1. Field Of The Invention
The present invention relates to Fourier-Transform (FT) Raman spectroscopy. More particularly, the present invention relates to a solid-state laser Fourier-Transform Raman spectrometer.
2. The Prior Art
A traditional FT Raman spectrometer incorporates a near-infrared Nd:YAG laser, a narrow bandwidth spectral filter for reducing spurious light from the laser, and a series of attenuators to adjust the laser output power over a wide range. The laser output is directed on to a sample and the light scattered from the sample is collected and the frequency content of the scattered light is analyzed.
The Raman emission is typically at very low light levels: roughly 1.times.10.sup.-8 of the incident power. Thus the FT Raman apparatus is very sensitive to spurious emission from the laser and to laser amplitude noise. One of the most common sources of spurious laser emission is the non-lasing fluorescent emission from the laser active medium and scattered light from the arc lamp or diode pump sources which are used to drive the laser. There are several strong emission lines located close enough to the desired laser line in Nd:YAG and it is thus difficult to remove them by filtering, with the result that they show up as spurious Raman signals. Common techniques to suppress these spurious signals are the use of dispersive elements such as grating or the use of absorbing interference filters. Both of these approaches tend to attenuate the laser beam by 50% to 75%, thus increasing the total amount of laser power required from the source.
The Raman spectrum is sensitive to fluctuations in the laser amplitude and wavelength. Fluctuations in laser amplitude will show up directly as noise on the Raman signal. Fluctuations in laser frequency will show up as noise in the location of the Raman line center. Unfortunately, the act of directing the laser onto a sample that scatters the incident light can increase the laser amplitude and frequency noise due to feedback of scattered light to the source laser.
One of the limiting factors in the frequency resolution of the FT Raman approach is the intrinsic linewidth of the laser system. Most diode pumped solid state lasers have emissions consisting of several longitudinal modes extending over several cm (about 50 GHz). This broadband emission often limits the achievable instrumental resolution.
Different samples have different Raman cross sections so it is necessary to attenuate the laser intensity to adjust the incident light to an appropriate level. Often this is done by turning down the drive power to the arc lamp or laser diode powering the laser source. Turning down the pump power has the negative side effects of changing the laser beam spatial profile, changing the emission direction of the laser beam, and altering the spectral profile of the emitting laser. All of these changes can impact the performance of the FT Raman instrument. Alternatively, neutral density filters that absorb or reflect a certain fraction of the laser power can be used which are mechanically inserted and removed as necessary. This approach requires the use of complicated moving parts which are subject to failure and relies on the attenuation of the neutral density filters remaining constant with time.
It is therefore an object of the present invention to provide a Fourier Transform Raman Spectrometer system which avoids or reduces these problems inherent in present systems.